Reduction of Chlorine Disinfection Dosage through Optimal Jet Design in the Hong Kong Harbor Area Treatment Scheme

Chlorine is commonly used in disinfection processes in wastewater treatment plants. The chlorine solution is typically dosed by turbulent mixing through multiport jet diffusers. The chlorine reacts with the inorganic and organic nitrogenous compounds at a fast rate (in the order of 1 s or less), and a significant portion of the dosed chlorine can be rapidly consumed by the reactions instead of destroying the pathogens. Despite extensive previous research, the chlorine consumption in the complex sewage flow remains an elusive problem. Field-scale experiments have been carried out to study the chlorine consumption in the turbulent mixing of dense chlorine jets with the primary treated sewage effluent of the Stonecutter’s Island Sewage Treatment Works (serving a population of 5.7 million). Based on jet theory and the field tests, it is proposed to discharge the chlorine jets at a location of highest velocity to minimize the contact time of the sewage with high concentration chlorine. Full-scale in-plant experiments demonstrated that significant savings in chlorine can be achieved with optimal jet diffuser design and placement; at nominal applied chlorine dosages of 12 and $20\mathrm{mg}/\mathrm{L}$ in winter and summer, respectively, the chlorine demand can be reduced by up to 30% compared with the conventional dosing design. The relatively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmful impact to the environment, and improves plant operation reliability. The chlorine demands from daily operations are well-correlated with the results of the field-scale models. Predictions of the chlorine demand as a function of applied dosage and temperature were obtained from the collective field data.